Re: [PATCH v2 5/7] btrfs: convert heuristic_collect_sample() to use folios

[David Sterba]

On Mon, Sep 07, 2026 at 04:19:59PM -0400, Tal Zussman wrote:
> Convert the sampling loop to folios. This removes the last caller of
> find_get_page() in btrfs and saves a call to compound_head() per sampled
> page. Document that the lookup is not supposed to fail with an ASSERT().
> 
> Reviewed-by: Qu Wenruo <wqu@xxxxxxxx>
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
> ---
>  fs/btrfs/compression.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
> index 979b2ffbd8fc..fa8b92592321 100644
> --- a/fs/btrfs/compression.c
> +++ b/fs/btrfs/compression.c
> @@ -1488,7 +1488,7 @@ static bool sample_repeated_patterns(struct heuristic_ws *ws)
>  static void heuristic_collect_sample(struct inode *inode, u64 start, u64 end,
>  				     struct heuristic_ws *ws)
>  {
> -	struct page *page;
> +	struct folio *folio;
>  	pgoff_t index, index_end;
>  	u32 i, curr_sample_pos;
>  	u8 *in_data;
> @@ -1514,8 +1514,10 @@ static void heuristic_collect_sample(struct inode *inode, u64 start, u64 end,
>  
>  	curr_sample_pos = 0;
>  	while (index < index_end) {
> -		page = find_get_page(inode->i_mapping, index);
> -		in_data = kmap_local_page(page);
> +		folio = filemap_get_folio(inode->i_mapping, index);
> +		ASSERT(!IS_ERR(folio));
> +		in_data = kmap_local_folio(folio,
> +				offset_in_folio(folio, (u64)index << PAGE_SHIFT));

It would be better to avoid the cast, this is source of subtle errors so
we try to avoid that. With demise of 32bit architectures it's less of a
problem because index (pgoff_t) is 64 bit type but still.

Please insert a patch that changes the type of index and index_end to
u64, we initialize them from a u64 type anyway and we know the value
fits to pgoff_t (as argument to filemap_get_folio()).